Add _sum for KernelSum{<:AbstractVector} and associated tests (#547)

* add sum for vectors and add tests for it

* Update test/kernels/kernelsum.jl

src/kernels/kernelsum.jl

@@ -45,6 +45,7 @@ Base.length(k::KernelSum) = length(k.kernels)
 
 _sum(f, ks::Tuple, args...) = f(first(ks), args...) + _sum(f, Base.tail(ks), args...)
 _sum(f, ks::Tuple{Tx}, args...) where {Tx} = f(only(ks), args...)
+_sum(f, ks::AbstractVector, args...) = sum(k -> f(k, args...), ks)
 
 (κ::KernelSum)(x, y) = _sum((k, x, y) -> k(x, y), κ.kernels, x, y)
 

test/kernels/kernelsum.jl

@@ -2,21 +2,24 @@
     k1 = LinearKernel()
     k2 = SqExponentialKernel()
     k = KernelSum(k1, k2)
-    @test k == KernelSum([k1, k2]) == KernelSum((k1, k2))
+    kvec = KernelSum([k1, k2])
+    @test k == kvec == KernelSum((k1, k2))
     for (_k1, _k2) in Iterators.product(
         (k1, KernelSum((k1,)), KernelSum([k1])), (k2, KernelSum((k2,)), KernelSum([k2]))
     )
         @test k == _k1 + _k2
+        @test kvec == _k1 + _k2
     end
-    @test length(k) == 2
-    @test repr(k) == (
+    @test length(k) == length(kvec) == 2
+    @test repr(k) ==
+        repr(kvec) ==
         "Sum of 2 kernels:\n" *
         "\tLinear Kernel (c = 0.0)\n" *
         "\tSquared Exponential Kernel (metric = Euclidean(0.0))"
-    )
 
     # Standardised tests.
     test_interface(k, Float64)
+    test_interface(kvec, Float64)
     test_interface(ConstantKernel(; c=1.5) + WhiteKernel(), Vector{String})
     test_ADs(x -> KernelSum(SqExponentialKernel(), LinearKernel(; c=exp(x[1]))), rand(1))
     test_interface_ad_perf(2.4, StableRNG(123456)) do c